1. Field of the Invention
The present general inventive concept relates to an image forming apparatus. More particularly, the present general inventive concept relates to an image forming apparatus which forms an image using a display apparatus, and a printing method using the same.
2. Description of the Related Art
An image forming apparatus is generally classified into a monochromatic type which forms black and white images, and a color type which forms color images. A color image forming apparatus is generally classified into a multi-pass type which rotates one image bearing member a plurality of times to form an image, and a single-pass type which rotates a plurality of image bearing members one time to obtain a color image.
First, the monochromatic type image forming apparatus will be explained briefly below.
With reference to FIG. 1A, a print medium (referred to as “paper” below) is picked up by a paper feeding part 11 of the main body 10, and fed between a developing machine 12 and an image transfer roller 13. A photosensitive medium such as a photosensitive drum 12a is housed in the developing machine 12, and an electrostatic latent image is formed on the photosensitive medium by the light beam emitted from a laser scanning unit (LSU) 14. A developer agent such a toner is supplied to the electrostatic latent image of the photosensitive drum 12a, and the developer image of the photosensitive drum 12a is transferred onto the paper sheet which is passed through the transfer roller 13 and the photosensitive drum 12a. The paper sheet, bearing the developer image thereon, is conveyed to a fusing part 15 where the image is fixed onto the paper sheet by the high temperature heat and high pressure. After exiting out of the fusing part 15, the paper may be discharged out, or returned through a paper reversing route, which is not shown, for image forming on the other side of the sheet, that is, for duplex printing.
FIG. 1B schematically illustrates the structure of a multi-pass type image forming apparatus.
Referring to FIG. 1B, a multi-pass type image forming apparatus is constructed to form a color image by rotating one image bearing member (referred to as a “photosensitive drum” below) several times. The multi-pass type image forming apparatus forms unit color images such as yellow, magenta, cyan, or black in turn and then transfers the unit color images onto a transfer medium such as transfer belt 27 where the unit color images are overlapped on another. The full color image of the transfer belt 27 is then transferred onto the paper sheet P. Therefore, one image bearing member 21 is rotated four times, and the transfer belt 27 is also rotated four times to form one full color image.
More specifically, in printing operation, K, C, M and Y developing machines 22, 23, 24, and 25, formed near to the photosensitive drum 21 respectively, form unit color images. That is, a Y color image is formed on the image bearing member 21 and transferred onto the transfer belt 27 by a first transfer roller 26 via a first transfer nip area N1 between the transfer belt 27 and the photosensitive drum 21. During this procedure, the second transfer roller 28 is at a distance from the transfer belt 27.
Next, an M color image is formed on the image bearing member 21, and transferred via the first transfer nip area N1 onto the transfer belt 27 now bearing the Y color image. Following the M color image, a C color image is transferred onto the transfer belt 26 already bearing the Y and M color images.
When a K color image is formed on the photosensitive drum 21 as the last unit color image, forming a full color image on the transfer belt 27, the second transfer roller 28 is moved to a position as indicated by dotted circles in FIG. 1B, creating a second transfer nip area N2 by contact with the transfer belt 27.
While the unit color images are being transferred onto the transfer belt 27, the paper sheet P picked up by the pickup roller from the paper feeding part 20 is fed toward the second transfer nip area N2.
Then, as the paper sheet P passes through the second transfer nip area N2, the full color image is transferred onto the paper sheet P.
The full color image is fused onto the paper sheet P by high temperature heat and high pressure as the paper sheet P passes through a fusing part 29. After passing through the fusing part 29, the paper sheet P is released.
Meanwhile, a laser scanning unit (LSU) 30 is needed to form an electrostatic latent image on the photosensitive drum 21, before the unit color images are formed on the photosensitive drum 21 by the color developing machines 22, 23, 24, 25 (FIG. 1A). The LSU 30 carries out a laser scanning operation in four cycles of a developing process to form the four color images, generally in the same manner as that of a monochromatic image forming apparatus.
In a single-pass type image forming apparatus, as shown in FIG. 1C, a plurality of image bearing members such as four photosensitive drums 41, 42, 43, 44 may be provided to form yellow, magenta, cyan and black color images. In this case, the color images of the four photosensitive drums 41, 42, 43, 44 are sequentially transferred onto the transfer medium such as the transfer belt 45 while the transfer belt 45 rotates once, and thus form a full color image. To this end, there are developing machines 51, 52, 53, 54 corresponding to the image bearing members 41, 42, 43, 44, and laser scanning units (LSUs) 60 corresponding to the photosensitive drums 41, 42, 43, 44.
Accordingly, it is possible to shorten the time to form a full color image on the transfer belt 45 to one rotation. After being transferred onto the transfer belt 45, the color image is again transferred onto a paper sheet P passing between the transfer roller 46 and the transfer belt 45. The paper sheet P bearing the color image is passed via the fusing part 47 and released.
The image forming apparatuses, whether they are monochromatic, single-pass, or multi-pass type, all use a laser scanning unit (LSU) to form an electrostatic latent image on the photosensitive medium. FIG. 2 shows one example of the LSU.
Referring to FIG. 2, a general LSU includes a light source 61 which emits a laser beam, a collimator lens 62 which changes the laser beam of the light source 61 into parallel rays or converging rays with respect to a light axis, a cylinder lens 63 which converges the parallel light rays only in a sub-scanning direction, a polygon mirror 64 which moves the laser beam passing through the collimator lens 62 and the cylinder lens 63 at a uniform linear velocity for scanning, a polygon mirror driving motor 65 which rotates the polygon mirror 64 at a uniform angular velocity, an F-theta lens 66 which has a refractive divergence with respect to the light axis and deflects the laser beam at a uniform velocity from the polygon mirror 64 in a main scanning direction, corrects aberration, and focuses the light beam onto a scanning surface, an image focusing reflective mirror 67 which reflects the laser beam passing the F-theta lens 66 such that the light beam is focused on the surface of a photosensitive medium, such as photosensitive drum 70, to form an electrostatic latent image, a sensor 68 which receives the laser beam, and a synchronous signal detection reflective mirror 69 which reflects the laser beam toward the synchronization sensor 68.
As explained above, conventionally, many optical parts of different functions are employed to form an electrostatic latent image on the photosensitive medium, and as a result, the LSU has the complicated structure.
The LSU 60 explained above, in particular, has a structure in which the image, in units of pixels, is formed by successive scan lines. Therefore, the image forming process is slow. Considering that there are increasing demands for high-speed printing and continuing attempts to increase the speed of the LSU, operating the driving motor 65 for rotating the polygon mirror 64, processing the image data, and operating the photosensitive medium and the developing machine in synchronization with each other, as in the LSU of the above structure, will be inefficient. In addition, synchronizing the parts with each other itself is a difficult process.
When one of the above components is slowed, it causes overall speed to decrease. It is particularly difficult to increase the rotational speed of the driving motor 65. More specifically, the driving motor 65 frequently has vibration during high-speed rotation, and it is difficult to precisely control the driving motor 65 to a uniform speed. The vibration can be reduced, but at the cost of reducing the rotational speed of the driving motor 65. Therefore, there is a limit to the printing speed.
Additionally, the light scanning direction at the surface of the of the photosensitive medium 70 is perpendicular to the rotation axis of the polygon mirror 64, and this causes lines of image data, in the electrostatic latent image, to form on the photosensitive medium 70 with skew. The technology to compensate for the skew is available, such as emitting the scanning light at an inclined angle to take the skew compensation value into account, but this requires a very complicated control mechanism, and still does not basically solve problems like skew. Particularly in the multi-pass or single-pass type image forming apparatus, the unit color images have to be in registration with each other in both the main and sub scanning directions, and problems such as these can interfere with registration.
The color images can be made in registration with each other by software, but exactly aligning the color images at the scale of one pixel is very difficult because of mechanical instability of the components related to the image formation, such as the photosensitive medium, the driving system, or the like. For example, although the manufacturer aims to form a clear-cut circular section when he makes the photosensitive drums, in practice it is hardly achievable, and therefore, errors are generated between the image data and this causes deterioration of image quality. Therefore, effort, time and costs are required to improve the mechanical precision and speed control of the components. However, there still is a limit to precise alignment of the image data in registration with each other.